Powder coating typically involves applying a composition that is in the form of solid particulates to a substrate, which is often then subjected to heat for curing. The heating process melts the particulate resin components and results in a continuous coating film. Such coatings are often used as a decorative and/or protective coating on appliances, automotive parts, electrical/mechanical equipment, furniture, among many other articles. In some cases, powder coating compositions are used to provide corrosion-resistant coatings, such as in subsurface pipeline applications.
Powder coatings have gained widespread use due to their durability over the product life cycle, economic benefits and safety/environmental advantages. For example, powder coating compositions do not include volatile organic solvents that are often used in liquid coating compositions, which is environmentally desirable. Also, powder coating application typically takes place in a controlled plant environment using electrostatic spray equipment in enclosed areas, which promotes adhesion and enables collection of overspray material for reuse, thereby limiting waste materials.
Most powder coating compositions include fillers or extenders that can constitute up to 40% by weight of a typical thermosetting powder coating formulation. Extenders are typically a very inexpensive ingredient in the composition that is used to extend the capabilities of the resin and coloring pigment(s) in the composition. They often are chemically inert in the composition and have little or no hiding power. Examples of commonly used extenders are talc, silicon dioxide, barium sulfate, calcium carbonate, wollastonite, calcium silicate, magnesium carbonate, micronized dolomite, and aluminum oxide.
Such extenders are known to have a detrimental effect on certain properties of powder coatings, particularly if used in too great an amount. For example, the smoothness, consistency and/or gloss level of the coating may be adversely affected. Also, the specific gravity of a powder coating composition can affect the coverage ability of the composition, with lower specific gravities resulting in higher film coverage on a square foot/pound coating/mil basis. Typical inorganic extenders have a relatively high specific gravity, resulting in limitations in terms of coverage.
In subsurface applications in particular, such as pipeline coating applications, it is important that the cured coating be resistant to a loss of adhesion to the substrate, which is sometimes referred to as “disbondment” of the coating material. This disbondment can be exacerbated by the use of cathodic protection that is commonly applied to buried pipes. Therefore, resistance to cathodic disbondment is an important property for pipeline coatings, especially those that consistently operate at elevated temperatures. If the coating fails to adequately adhere to the pipe, the corrosion-resistance properties of the coated substrate are lost.
As a result, it would be desirable to provide powder coating compositions that include an extender that, unlike conventional inorganic extenders, is an organic material with functional properties not normally found in inorganic extenders. Moreover, because they are often used in applications in which protection against metal corrosion is important, it would be desirable to provide such powder coating compositions that result in cured coatings with good corrosion resistance properties and which are resistant to cathodic disbondment.
The present invention was made in view of the foregoing.